1. Field of Invention
Aspects of the invention relate to the shape of a piezoelectric resonator element for preventing a thickness shear vibration, which is the main vibration, from being propagated to the outer peripheral portion of the piezoelectric resonator element.
2. Description of Related Art
Regarding a thickness shear vibration, which is the main vibration of an AT cut quartz crystal resonator, although the placement of an excitation electrode is decided so that the center portion of the resonator element oscillates, the thickness shear vibration oscillating in the center portion can be propagated to the outer peripheral portion of the resonator element.
The resonator element is held by fixing the outer peripheral portion of the resonator element to a housing made of ceramic or the like, and the propagated vibration can be suppressed because of fixing. This suppression of the propagated vibration has an effect on the thickness shear vibration, which is the main vibration. As a result, the crystal impedance (referred to as “CI value” hereinafter) decreases and other vibration modes are induced to decrease the stability of oscillation frequency.
Related art methods for suppressing propagation of the thickness shear vibration to the outer peripheral portion of the resonator element by providing a groove that surrounds an excitation electrode placed on the main surface of the resonator element and is located between the excitation electrode and the outer peripheral portion of the resonator element to dampen the vibration are disclosed, for example, in Japanese Unexamined Patent Publication No. 2001-257558 and Japanese Unexamined Patent Publication No. Hei9-93076. Incidentally, the patents do not provide any disclosure on the depth, shape, and location of the groove.